Geothermal energy resources have generated considerable interest in recent years as an alternative to conventional hydrocarbon fuel resources. Fluids obtained from subterranean geothermal reservoirs can be processed in surface facilities to provide useful energy of various forms. Of particular interest is the generation of electricity by passing geothermal steam or vapor through a steam turbine coupled to an electric generator.
Several different types of geothermal power plants are known. These include, for example, direct cycle plants, flash steam plants, indirect cycle plants, binary cycle plants, and combined or hybrid plants. Direct cycle plants, which are of particular interest with regard to the current invention, include a steam turbine that is driven. directly by steam from the earth's interior. The steam after being expanded in the turbine is condensed in a condenser and released into the atmosphere or re-injected into subterranean formations.
The U.S. Pat. No. 5,925,291 describes a direct contact condenser for geothermal applications. Geothermal fluids typically comprise a variety of potential pollutants, including noncondensable gases (NCG) such as ammonia, hydrogen sulfide, and methane. Because of these contaminants, particularly hydrogen sulfide, discharging a geothermal vapor exhaust into the atmosphere is usually prohibited for environmental reasons. Thus, the conventional approach is to exhaust the turbine effluent into a steam condenser to reduce the turbine back pressure and concentrate the noncondensable gases for further downstream venting, treatment or elimination.
THE '291 patent further suggests that many geothermal power plants utilize direct contact condensers, wherein the cooling liquid and vapor intermingle in a condensation chamber, to condense the vapor exhausted from the turbine. Direct contact condensers are generally preferred over surface condensers in the case of vapor condensation with high content of non-condensable gases with corrosion potential. In the surface condensers the vapor releases its condensation heat to the circulating cooling water across a separation wall. This type of condensers is the preferred realisation of a cycle heat sink due to the excellent overall mean heat transfer coefficient obtainable for condensing pure (or quasi-pure) vapors in surface condensers.
However, for condensing steam with high non-condensable content (e.g. greater than 0.5% of mole fraction), the use of l the less efficient direct contact condenser are considered because of the gas film boundary layer, which increase enormously the thermal resistance for the heat transfer. To realize an optimal heat transfer efficiency using direct contact condensers, the cooling liquid must be introduced into the condensation chamber at a sufficiently high velocity to disperse the liquid into fine droplets, i.e., to form a rain, thereby increasing the surface area for condensation.
Unfortunately, this high velocity discharge reduces the contact time between the cooling liquid and the vapor, which in turn reduces the heat exchange efficiency. Consequently, conventional direct contact condensers require relatively large condensing chambers to compensate for this low heat transfer efficiency and to provide sufficient contact between the liquid and vapor to effect condensation.
As stated in the '291 patent, a possible way to increase the condensation efficiency, and thus to minimize the size of the direct contact condenser, is to inject the cooling liquid through a plurality of individual nozzles, which disperses the cooling liquid in the form of droplets or films. As films or droplets provide a greater surface area for condensation than normal liquid injection, the cooling liquid can be introduced into the chamber at a lower rate, i.e., without generating a rain of fine droplets. Although these spray-chamber condensers offer generally improved condensation efficiency and more compact designs than previous generation of condensers, they require substantial quantities of cooling liquid to obtain sufficient condensation. Therefore, and because of the additional energy requirements and losses associated with pumping the excess cooling liquid to the condensation chamber, the practical efficiency of these condensers remains still low.
The U.S. Pat. No. 3,814,398 discloses a direct contact condenser having a plurality of spaced-apart deflector plates angularly disposed relative to the cooling liquid inlet. The deflector plates are positioned to break up the cooling liquid into liquid fragments, thus generating a film of coolant. The condenser includes multiple spray chambers, wherein each chamber has deflector plates and a conduit for a liquid. Obvious disadvantages of this design are its complexity and high costs due to the large numbers of partitions, deflector plates, and liquid conduits required to generate the film.
The condenser described in the U.S. Pat. No. 5,925,291 has a downward vapor flow chamber and an upward vapor flow chamber, wherein each of the vapor flow chambers includes a plurality of cooling liquid supplying pipes and a vapor-liquid contact medium disposed thereunder to facilitate contact and direct heat exchange between the vapor and cooling liquid. The contact medium includes a plurality of sheets arranged to form vertical interleaved channels or passageways for the vapor and cooling liquid streams. The upward vapor flow chamber also includes a second set of cooling liquid supplying pipes disposed beneath the vapor-liquid contact medium which operate intermittently in response to a pressure differential within the upward vapor flow chamber. The condenser further includes separate wells for collecting condensate and cooling liquid from each of the vapor flow chambers. In alternate embodiments, the condenser includes a cross-current flow chamber and an upward flow chamber, a plurality of upward flow chambers, or a single upward flow chamber.
While providing an efficient cooling system, the condenser described in the '291 patent can often be difficult to manufacture and to maintain as it is challenging to form the interleaved channels from steel. The channels are equally not easy to clean in order to prevent fouling or scaling. It can therefore be seen as an object of the present invention to provide a compact and efficient direct contact condenser, which avoids the disadvantageous of the known cooling methods, particularly as applied to condensate steam from geothermal sources.